The invention relates to a method of converting a first pseudo-halftone picture into a second pseudo-halftone picture such that n.sup.2 picture elements of the first picture correspond to m.sup.2 elements of the second picture, where m.noteq.n and preferably m &lt;n. The first pseudo-halftone picture is formed by a matrix of binary picture elements whose values are obtained in an ordered dither way from the scanning of an original document and comparing the picture elements arranged in a matrix of i lines and j columns to a set of different threshold values which are also arranged in a matrix having also i lines and j columns. The invention also relates to a circuit arrangement for performing the method.
The ordered dither method is, for example, described in Bell System Techn. Journal 48, 1969, pages 2555-2582 and is used to display, with the aid of matrix printers or screen display devices which can only display picture elements of equal sizes and intensity, pseudo-halftone pictures with good quality. For completeness, this known method will now be described in greater detail with reference to FIG. 1. Therein the subpicture 20 illustrates a portion of a scanned halftone picture having a size of four x four picture elements, the numbers incorporated in these picture elements indicating the brightness value or grey value of the relevant picture element. The higher a value the darker the picture element, so that it will be clear that in this subpicture 20 the picture elements taken on a diagonal from bottom left to top right become darker. In order to produce a binary pseudo-halftone picture therefrom, the subpicture 20 is compared with a matrix 21 of threshold values. When the brightness value or grey tone of a picture element exceeds the threshold value indicated in the corresponding picture element position of the matrix 21, a dot is recorded. The binary picture elements thus obtained are shown in the subpicture 22. Actually, the subpicture 20 or 22 are so small that in the pseudo-halftone picture the individual picture elements merge when they are observed at a normal viewing distance and a grey tone impression is obtained, which depends on the number of black picture elements in each subpicture 22.
The binary pseudo-halftone pictures obtained with the aid of the ordered dither method described in the foregoing can now be transmitted or stored. The reproduction is effected with devices which have the same resolution, that is to say the same number of elements per mm, when it is a requirement for the picture size to be maintained.
If one wants to reproduce such a transmitted or stored picture using a device having a different resolution, that is to say a different number of elements per mm, the picture size remaining the same, the number of picture elements must be increased when a higher resolution is required or decreased when a lower resolution is required. When this is effected such that uniformly spaced-apart picture lines and picture columns are suppressed or doubled, depending on whether an increase or a decrease of the resolution is required, the optical impression is degraded, at least at given ratios between the change in the resolution and predetermined values or configuration of the threshold values in the matrix 21, as, more specifically when picture lines and columns are omitted, when always those lines and columns are omitted which belong to the same lines and columns of the matrix 21, the corresponding halftones are lost. In addition, the change in resolution by the superpositioning of several frames may result in coarse structures.